There exist various types of color display devices are available for commercial use. Thin-type display devices are divided into two main categories: self-luminous types like the PDP (plasma display panel) and non-luminous types like the LCD (liquid crystal display). A well-known subcategory of the LCD, a typical example of non-luminous type display devices, is the transmissive LCD containing a backlight behind a liquid crystal panel.
FIG. 16 is a cross-sectional view of a general structure of the transmissive LCD. The transmissive LCD includes a backlight 110 behind a liquid crystal panel 100. The liquid crystal panel 100 has a liquid crystal layer 103 between a pair of transparent substrates 101, 102. Polarizers 104, 105 are provided outside the transparent substrates 101, 102. The liquid crystal panel 100 has provided therein a color filter 106 to produce a color display.
The transparent substrate 101, 102 has provided therein an electrode layer and orientation films (not shown) so that the amount of light passing through the liquid crystal panel 100 can be controlled in terms of individual pixels by controlling application voltage to the liquid crystal layer 103. In other words, the transmissive LCD controls the display by controlling transmission through the liquid crystal panel 100 of the light emitted by the backlight 110.
The backlight 110 projects light including three wavelengths (RGB) necessary for the color display device. The backlight 110, used in conjunction with the color filter 106 to regulate individual transmittances for RGB light, enables arbitrary settings of luminance and hue as pixels. The backlight 110 is typically an electroluminescent (EL) lamp, a cold cathode fluorescent lamp (CCFL), a light emitting diode (LED), or another white light source.
In the liquid crystal panel 100, pixels are arranged to form a matrix with each pixel normally being composed of three subpixels as illustrated in FIG. 17. The subpixels are arranged to match a red (R), a green (G), or a blue (B) filter layer in the color filter 106. The subpixels will be referred to as R subpixels, G subpixels, and B subpixels throughout the following.
The R, G, B subpixels selectively transmit particular wavelengths (i.e., red, green, and blue) of the white light produced by the backlight 110 and absorb other wavelengths.
In the transmissive LCD constructed as above, the transmission through the liquid crystal panel 100 of the light emitted by the backlight 110 is controlled by the pixels in the liquid crystal panel 100; some of the light is inevitably absorbed by the liquid crystal panel 100. In addition, the R, G, B subpixels in the color filter 106 absorb the white light produced by the backlight 110 except for the particular wavelengths. These facts demonstrate that the liquid crystal panel and the color filter absorb much of incoming light in ordinary transmissive LCDs. That results in a low use efficiency for the light emitted by the backlight, which in turn leads to a problem of large power consumption by the backlight.
A known technique for reducing the power consumption by the transmissive LCD is a method involving use of an active backlight of which the luminance is adjustable according to an image being displayed. The technique is disclosed, for example, in Japanese Unexamined Patent Publication No. 65531/1999 (Tokukaihei 11-65531, published Mar. 9, 1999; “Patent Document 1”).
Patent Document 1 employs an active backlight of which the luminance is adjustable and attempts to lower the power consumption by the backlight by controlling the display on the LCD (luminance control) through the control of the transmittance of the liquid crystal panel and the luminance of the active backlight.
In Patent Document 1, the luminance of the backlight is controlled to match a maximum luminance value of an input image (input signal). The transmittance of the liquid crystal panel is adjusted according to the thus controlled luminance of the backlight.
Under these conditions, the subpixels corresponding to the maximum input signal level exhibit a 100% transmittance. The other subpixels show 100% or lower transmittances as calculated in reference to the luminance of the backlight. Therefore, when the whole image is dark, the backlight is dimmed to reduce the power consumption by the backlight.
In this manner, in Patent Document 1, the brightness of the backlight is reduced to a minimum according to the input RGB signals for the input image. Meanwhile, the absorption of light by the liquid crystal panel is reduced by increasing the transmittance of the liquid crystal as much as the backlight is dimmed. The structure hence reduces the power consumption by the backlight.
The conventional structure indeed reduces the power consumption by the backlight by reducing the absorption by the liquid crystal panel. The structure however is unable to reduce the absorption of light by the color filter. If the absorption by the color filter is reduced, the power consumption is further reduced.